Transdermal drug delivery is a highly attractive mode for reasons that include patient convenience, compliance and comfort, avoidance of irritation to the gastrointestinal tract, bypass of hepatic and gastrointestinal metabolism, and reduction of fluctuations in drug plasma. However, a key challenge in transdermal drug delivery is the penetration of several barriers in the skin and underlying tissues to achieve systemic or localized drug deposition.
If transdermal delivery is desired, the first barrier to overcome is the outer layer of the skin, the epidermis. The outer layer of skin is made up of living and dead epithelial cells that are made in the stratum basale and are continually pushed to the surface as new cells are made. The cells are initially alive but eventually metabolism ceases and the cells undergo programmed death becoming flattened and keratinized. These dead cells create the outermost layer of the epidermis, the stratum corneum. It is the stratum corneum that is the principal barrier to the percutaneous penetration of topically applied drugs.
The dermis is the layer of skin beneath the epidermis and is separated from the epidermis by a basement membrane. The dermis consists mainly of connective tissue; nerve endings; sweat, sebaceous, and apocrine glands; hair follicles; and blood vessels. The blood vessels in the dermis release plasma containing water, nutrients and oxygen into the interstitial spaces and channels of the epidermis and dermis to provide nourishment to these tissues and, in a separate but continuous action, re-absorb interstitial fluid back into the capillaries and lymphatic vessels thereby removing cellular waste products and water.